1. Field of the Invention
The present invention relates to a dielectric thin film capacitor element used for electronic parts such as an IC capacitor, and a nonvolatile memory device and to a method of manufacturing the same.
2. Description of Related Art
Hitherto, Ta.sub.2 O.sub.5 (tantalum pentoxide), SiO.sub.2 (silicon dioxide) and SiN (silicon nitride) have been mainly used as a dielectric material of a dielectric thin film capacitor for an IC typified by a signal accumulating capacitor of a DRAM and a capacitor for a microwave device of a MMIC (Microwave Monolithic Integrated Circuit). However, with the recent miniaturization and high-integration of electronic parts along the progress of the semiconductor technology, an extra-thin dielectric thin film or three-dimensional dielectric thin film has come to be fabricated in order to reduce capacitor area. For this reason, a process for fabricating semiconductor devices has become more and more complicated and the micro-processing technology has come close to its limit, causing a problem in yield, reliability and the like. Then, a dielectric thin film having a higher dielectric constant as compared to that of the conventional dielectric thin film has become necessary. Presently, a research on an oxide high dielectric thin film made of SrTiO.sub.3 (strontium titanate) or (Ba, Sr)TiO.sub.3 (barium strontium titanate) having a higher dielectric constant as compared to SiO.sub.2 and SiN described above is being actively conducted.
However, the dielectric thin film capacitor element using the conventional oxide high dielectric thin film such as SrTiO.sub.3 and (Ba, Sr)TiO.sub.3 described above has had a big problem in its reliability. That is, no high dielectric thin film capacitor element which provides practical enough characteristics has been realized yet in a high temperature energizing test, which is a usual reliability test for electronic parts, in which a certain voltage is applied to a capacitor while holding the capacitor at a certain temperature. For example, in a high temperature energizing test of bias application of 10 V to the capacitor while holding the capacitor at 100.degree. C., leak current increases by three to four digits in about 10 hours, causing a problem of degradation of resistance of the capacitor such that it cannot maintain the insulation required as the capacitor.
The degradation of the resistance of the capacitor is considered to be caused by the following reason. In the oxide high dielectric thin film such as SrTiO.sub.3 and (Ba, Sr)TiO.sub.3, a lattice defect of oxygen (oxygen vacancy) occurs within the thin film during its growth. Because the oxygen vacancy is electrified to +bivalent in a capacitor element composed of the dielectric thin film containing the oxygen vacancy, the oxygen vacancy moves to the cathode side when the capacitor element is heated and voltage is applied between electrodes thereof during the high temperature energizing test. Then, the oxygen vacancy which has moved to the cathode side cannot move to the cathode due to a potential barrier of the cathode and SrTiO.sub.3 or (Ba, Sr)TiO.sub.3 and it increases at the boundary of the cathode and the dielectric thin film. At this time, electrons are injected from the cathode side to electrically compensate for it. Further although oxygen vacancy is introduced newly from the anode side, the electrons are generated as a carrier at this time. Due to these phenomena, the conductivity increases as time passes by and leak current increases in the dielectric thin film as a whole.
It is conceivable to implement an oxygen annealing treatment or oxygen plasma treatment in which heat treatment is implemented within an oxygen atmosphere after forming the film in order to compensate for the oxygen vacancy and to prevent the capacitor from degrading. However, even if such treatment is implemented, it is difficult to reduce the oxygen vacancy after forming the dielectric thin film because oxygen is not stably incorporated into a dielectric crystal lattice in terms of thermal equilibrium.
As a prior art technology, an experimental example in which a characteristic time tch until the occurrence of a dielectric breakdown could be prolonged, when energized in high temperature, by doping La.sub.2 O.sub.3 to a bulk ceramic sintered body of BaTiO.sub.3 is described in J. B. MacCHESNEY et al., J. Am. Ceram. Soc., 46(1963)197. Specifically, it is reported that while the time until the occurrence of the dielectric breakdown was 2 hours in one to which no La.sub.2 O.sub.3 has been doped, it increased up to 1000 hours or more in one to which 0.50 mol % of La.sub.2 O.sub.3 has been doped.
Further, R. Waser et al., J.Am.Ceram.Soc., 73(1990) 1645 describes an experimental case in which the increase of leak current was suppressed, when energized in high temperature, by doping 0.3 mol % of La to a SrTiO.sub.3 bulk ceramic sintered body.
Further, Japanese Unexamined Patent Publication JP-A 6-326250 (1994)(TI) describes a case in which (Ba, Sr)TiO.sub.3 to which 0.25 mol % or more of La, Er and the like has been doped is used as a high dielectric material.
Although all of the prior art technologies described above use the bulk ceramic sintered body to which La is doped as a dielectric material, it is difficult to contain a desired content of La in the process for forming this film. Further, although Japanese Unexamined Patent Publication JP-A 6-326250 (1994) describes about the use of Er, of which content is set at 0.25 mol % or more, the time until the occurrence of a dielectric breakdown is short when the content is 0.25 mol % or more.